1. Field of the invention
This invention relates to a new silacyclohexane compound, a method of preparing it, and a liquid crystal composition which contains it, as well as a liquid crystal display element which contains said liquid crystal composition.
2. The Prior Art
A liquid crystal display element utilizes the optical anisotropy and dielectric anisotropy of liquid crystal substances. Display methods include the TN mode (twisted nematic mode), the STN mode (super twisted nematic mode), the SBE mode (super birefringence mode), the DS mode (dynamic scattering mode), the guest-host mode, the DAP mode ("deformation of aligned phase" mode) and the OMI mode (optical mode interference mode). The most common display device has a twisted nematic structure based on the Schadt-Helfrich mode.
The properties required of the liquid crystal substance used in these liquid crystal displays are somewhat different depending on the display method. However, a wide liquid crystal temperature range and stability with regard to moisture, air, light, heat, electric fields, etc., are properties commonly required in all display methods. Furthermore, it is desirable for the liquid crystal material to have a low viscosity, and also to have a short address time, low threshold voltage and high contrast in the cell(s).
Currently, there is no single compound which satisfies all these requirements. In practice, liquid crystal mixtures are obtained by mixing several to more than ten liquid crystal compounds and latent liquid crystal compounds. Because of this, it is also important that components of a liquid crystal composition mix easily.
Of these components, the following compounds with the cyclohexyltolane ring structure have been known as compounds which have a high .increment.n (anisotropy of the refractive index) and a relatively high T.sub.NI (nematic-isotropic transition temperature). ##STR3## (See Japanese unexamined patent publication Tokkai Sho 63-310838.) ##STR4## (See Japanese examined patent publication Tokko Sho 64-3852.) ##STR5## (See Tokko Hei 1-34976.) ##STR6## (See Tokko Hei 2-53415.) ##STR7## (See Tokko Hei 3-67057.) ##STR8## (See Tokko Hei 5-2656.)
The following compounds have been known as compounds which have a high .increment.n, a relatively high T.sub.NI and also a negative .increment..di-elect cons. (dielectric anisotropy). ##STR9## (See Japanese unexamined patent publication Tokuhyo Hei 1-502908.) ##STR10## (See Tokkai Hei 3-145450.)
In recent years, along with the expansion of the applications of liquid crystal displays, panel operation modes are diversified and the characteristics required of liquid crystal materials are becoming more and more advanced. The most widely used operation modes are the TN mode with the TFT drive method and the STN mode. Since STN is inexpensive, its display performance is expected to be improved and its applications then widened. There is particularly great demand for faster response time and improved contrast.
One of the ways to shorten the response time is to make the cell gap thinner. In this case, because of the retardation limitation (d.times..increment.n, d: cell gap, .increment.n: anisotropy of the refractive index), liquid crystal compositions with a larger .increment.n are required.
In order to obtain a high contrast, a steep threshold characteristic, i.e. a small dielectric constant ratio (.increment..di-elect cons./.di-elect cons., .increment..di-elect cons.: dielectric anisotropy, .di-elect cons.: dielectric constant along the minor axis) is required. Most of the liquid crystal compounds have a positive or near-zero .increment..di-elect cons., and therefore a liquid crystal compound with a negative .increment..di-elect cons. is required to decrease the dielectric constant ratio of mixed compositions using them.